Methods and apparatus for a voltage-shifting readout circuit

ABSTRACT

Methods and device for a readout circuit according to various aspects of the present invention may operate in conjunction with a storage device selectively coupled to an input signal having a voltage value within a first voltage range. A comparator may compare the voltage value of the input signal to a predetermined threshold voltage. A level-shifting circuit may shift the first voltage value of the input signal to a second voltage value within a second voltage range if the first voltage value of the input signal is greater than the predetermined threshold voltage.

BACKGROUND OF THE TECHNOLOGY

Image sensors utilize analog-to-digital converters (ADCs) to convert theanalog pixel signal into a digital signal prior to signal processing. Inconventional column readout architectures, each column of the pixelarray has a dedicated readout circuit and corresponding ADC. The readoutcircuit typically includes a sample and hold circuit using a capacitorand a buffer, where the pixel signal is sampled prior to being convertedinto a digital signal. The maximum voltage for the readout signal may behigh (e.g., 3.3V) due to the pixel output swing. As such, the inputswing for the ADC must match that of the pixel output swing.Conventional circuits utilize amplifiers to increase the pixel outputswing. Matching the high voltage results in the ADC operating at ahigher voltage and therefore requires more power, which generally addsto the cost of the device. Other readout circuits have been designed toattenuate the signal, however, signal attenuation results in the loss ofinformation, which introduces bit errors when the signal is converted toa digital signal.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

A more complete understanding of the present technology may be derivedby referring to the detailed description when considered in connectionwith the following illustrative figures. In the following figures, likereference numbers refer to similar elements and steps throughout thefigures.

FIG. 1 is a block diagram of a readout circuit in accordance with anexemplary embodiment of the present technology;

FIG. 2 representatively illustrates various voltage ranges for a readoutcircuit in accordance with an exemplary embodiment of the presenttechnology;

FIGS. 3A-B are schematic drawings of alternative arrangements of areadout circuit in accordance with an exemplary embodiment of thepresent technology;

FIG. 4A representatively illustrates a transfer function of aconventional readout circuit;

FIG. 4B representatively illustrates a transfer function of a readoutcircuit in accordance with an exemplary embodiment of the presenttechnology;

FIG. 5 is a schematic of a readout circuit in accordance with anexemplary embodiment of the present technology;

FIG. 6 representatively illustrates a timing diagram in accordance withan exemplary embodiment of the present technology;

FIG. 7 is a schematic of a readout circuit in accordance with anexemplary embodiment of the present technology;

FIG. 8 representatively illustrates a timing diagram in accordance withan exemplary embodiment of the present technology; and

FIG. 9 representatively illustrates a system in accordance with anexemplary embodiment of the present technology.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present technology may be described in terms of functional blockcomponents and various processing steps. Such functional blocks may berealized by any number of components configured to perform the specifiedfunctions and achieve the various results. For example, the presenttechnology may employ various semiconductor devices, such astransistors, capacitors, and the like, which may carry out a variety offunctions. In addition, the present technology may be practiced inconjunction with any number of imaging systems, and the apparatusdescribed is merely one exemplary application for the technology.Further, the present technology may employ any number of conventionaltechniques for capturing light and converting the light into an electriccharge, reading out the charge as a pixel signal with a voltage value,sampling the voltage of the pixel signals, converting the analog pixelsignal into a digital signal, and the like.

Methods and apparatus for a readout circuit according to various aspectsof the present technology may operate in conjunction with any suitableimaging system, such as a computer system, camera system, machine visionsystem, vehicle navigation system, video telephone, surveillance system,auto focus system, star tracker system, motion detection system, imagestabilization system, smartphone, and the like. The readout circuit 100may accumulate charge generated by one or more pixels and transfer theresultant signal to an ADC for conversion to a digital value.

Referring now to FIG. 9, an exemplary imaging system according tovarious aspects of the present invention may comprise an electronicdevice, such as a digital camera 900. The imaging system may comprise acentral processing unit (CPU) 910 that communicates with various devicesover a bus 915. Some of the devices connected to the bus 915 may providecommunication into and out of the system, for example an input/output(I/O) device 920. Other devices connected to the bus 915 provide memory,for example, a random access memory (RAM) 925, hard drive, and one ormore removable memory devices 930, such as a floppy disk drive, compactdisk (CD) drive, USB drives, memory cards and SD cards. While the bus915 is illustrated as a single bus, any number of busses may be used toprovide communication paths to interconnect the devices.

The imaging system may further comprise an imaging device 905 forcapturing image data. The imaging device 905 may comprise a plurality ofpixels, for example arranged in rows and columns to form a pixel array.In an exemplary embodiment, each pixel comprises a photodetector, suchas a photodiode, for capturing light and converting light into anelectric signal. The signal produced by the pixel is an analog signalwhich may be converted into a digital signal prior to signal processing.As such, the imaging device 905 may employ one or more analog-to-digitalconverters (ADC) 125 (FIG. 1). For example, in a per-column ADC, eachcolumn of the pixel array has an associated ADC, while otherarchitectures (i.e., chip level) may comprise only one ADC. Prior todigital conversion, the pixel signal may undergo sampling, shifting, andthe like.

In various embodiments, the imaging device 905 may be combined with aprocessor, such as the CPU 910, a digital signal processor, or amicroprocessor, in a single integrated circuit, or may be on a separatechip. The imaging device 905 may be a CMOS imager constructed inaccordance with any of suitable technology and/or control methods. Theimaging device 905 may also receive control or other data from thesystem as well, such as a shutter release.

In various embodiments, the electronic device may further comprise alens 935 configured to focus an image on the pixel array. For example,the lens 935 may include a fixed and/or adjustable lens and may includemicrolenses formed on an imaging surface of the imaging device 905.

Referring now to FIG. 1, in an exemplary embodiment of the presenttechnology, the readout circuit 100 may operate in conjunction with theADC 125, in this case a delta-sigma modulator. While the exemplaryreadout circuit 100 operates in conjunction with a delta-sigmamodulator, any suitable ADC may be used. In the present embodiment, thereadout circuit 100 receives an analog pixel signal Vin_pix from thepixel array at an input, and the readout circuit 100 may sample, shift,and transmit corresponding signals to the ADC 125. In variousembodiments, the output of the ADC 125 may be transmitted to a dataformatting unit or other system for further processing and/or filtering.

The readout circuit 100 may comprise a comparator 110 and alevel-shifting circuit 115. The comparator 110 compares the pixel signalvalue to a reference value and generates a corresponding signal if thepixel value exceeds the reference value. The comparator 110 may compriseany appropriate system or device for comparing the pixel signal value tothe reference value, such as a conventional comparator circuit havingtwo voltage inputs and a voltage output. For example, the exemplarycomparator 110 of the present embodiment may comprise two inputterminals (i.e., first and second input terminals) and one outputterminal. The comparator 110 compares the input voltages and generatesan output signal indicating which of the two input voltages is larger.In an exemplary embodiment, the comparator 110 receives the input pixelvoltage Vin_pix at the first terminal and a threshold voltage at thesecond terminal (not shown), such as a static predetermined voltagevalue. If the input pixel voltage Vin_pix is larger than the thresholdvoltage, the comparator 110 outputs a digital “1.” Conversely, if theinput pixel voltage Vin_pix is less than the threshold voltage, thecomparator 110 outputs a digital “0.” In an exemplary embodiment, thecomparator 110 output Vout_comp is transmitted to the level-shiftingcircuit 115 and to the data formatting unit (not shown). In otherembodiments, the comparator 110 may be configured as a multi-levelcomparator 110 for multi-bit comparison.

In various embodiments, the comparator 110 may provide a mostsignificant bit signal to the data formatting unit according to theinput signal.

The level-shifting circuit 115 generates a signal corresponding to thepixel signal value and responds to the comparator 110. If the comparator110 output signal indicates that the pixel signal value exceeds thethreshold, the level-shifting circuit 115 shifts the value of its outputsignal to a smaller value.

In the present exemplary embodiment, the level-shifting circuit 115 maycomprise any suitable circuit and/or semiconductor device for receivingan input voltage Vin_level at an input 145 and selectively producing acorresponding but shifted output voltage Vout_level at an output 140,such as a lower output voltage than the input voltage Vin_level.

Various embodiments of the level-shifting circuit 115 may compriseswitching devices, such as transistors, configured to receive thecomparator output Vout_comp, wherein the switching device mayselectively couple to one or more reference potentials. In variousembodiments, the level-shifting circuit receives the input voltageVin_level corresponding to a pixel voltage value at a given time, andoutputs the voltage Vout_level, wherein the maximum voltage value of theoutput signal Vout_level_max is less than the maximum voltage value ofthe input signal Vin_level_max.

In other embodiments, the level-shifting circuit 115 may be configuredas a multi-level level-shifting circuit to operate with a multi-levelcomparator 110.

The readout circuit 100 may further comprise a storage device 105 forstoring a voltage value. The storage device may be coupled to the pixelinput Vin_pix. For example, the storage device 105 may comprise acapacitive element that provides memory through a stored electrostaticfield, such as a conventional capacitor.

The storage device 105 may be utilized in conjunction with a switch tosample the voltage of the input signal Vin by storing electric charge.For example, the readout circuit 100 may further comprise a firstswitching device 130 for selectively connecting the input signal to thestorage device 105. The first switching device 130 may receive a controlsignal from a control unit (not shown). The first switching device 130responds by “opening” and “closing,” wherein “open” may be defined asinhibiting current flow, and “closed” may be defined as allowing currentflow. For example, the first switching device 130 may comprise a deviceresponsive to a control signal, such as a transistor responsive to acontrol signal or any other suitable device to selectively facilitatecurrent flow.

The readout circuit 100 may further comprise a buffer 120 fortemporarily storing data. In an exemplary embodiment, the buffer 120 mayreceive an input Vin_buffer at a positive input terminal 150 whichcorresponds to the level-shifting circuit output Vout_level, andproduces an output Vout_buffer. The buffer output Vout_buffer may betransmitted to the ADC 125.

The readout circuit 100 may further comprise a second switching device135 for selectively connecting the output 140 of the level-shiftingcircuit 115 to the input of the positive input terminal 150 of thebuffer 120. The second switching device 135 may receive a control signalfrom the control unit. The second switching device 135 responds by“opening” and “closing” accordingly. For example, the second switchingdevice 135 may comprise a device responsive to a control signal, such asa transistor responsive to a control signal or any other suitable deviceto facilitate current flow.

In various embodiments, the readout circuit 100 operates to reduce thevoltage swing (i.e., range) of the input signal without attenuating thesignal. In an exemplary embodiment, the storage device 105 samples thevoltage of the input signal. The comparator 110 receives the sampledvalue at the first input terminal and the threshold voltage at thesecond input terminal. The comparator 110 compares the sampled value andthe threshold value Vth. The comparator 110 then outputs a binary valuerepresenting the outcome of the comparison. The comparator 110 outputVout_comp is then transmitted to the level-shifting circuit 115 and tothe data formatting unit (not shown). The level-shifting circuit 115responds to the comparator output Vout_comp, such that if thelevel-shifting circuit 115 receives a binary “1,” then thelevel-shifting circuit 115 activates to shift the voltage to a new value(i.e., a shifted value). The shifted value is then transmitted to thebuffer 120 where it is temporarily stored prior to transmission to theADC. This process of sampling, comparing, selectively activating thelevel-shifting circuit 115, and storing the new value continues untilthe entire pixel array has been read out.

The readout circuit 100 may be arranged in any manner suitable togenerate an output signal having a reduced range while maintainingprecision of the signal, such as shifting the level of the output signalin response to a comparison of the input voltage to one or morethresholds and/or generating additional bits or other informationaccording to the level shifting operations. FIG. 1 illustrates oneembodiment of the readout circuit 100, while FIGS. 3A-B illustratealternative embodiments of the readout circuit 100.

Referring now to FIG. 2, in operation, the readout circuit 100 maygenerate various voltage ranges via the level-shifter 115 outputVout_level, the buffer 120 output Vout_buffer, and the ADC 125 inputVin_adc in relation to the pixel input Vin. In an exemplary embodiment,the pixel input Vin_pix comprises a voltage range from a minimum voltageVmin_pix to a maximum voltage Vmax_pix. The level-shifter 115 produces amaximum output voltage Vout_level_max which is less than the maximumpixel input voltage Vmax_pix. The input voltage range of the buffer 120(i.e. Vin_buffer min to Vin_buffer max) may be equal to, orsubstantially equal to, the output range of the level-shifter 115 (i.e.,Vout_level_min to Vout_level_max). Similarly, the output voltage rangeof the buffer 120 (i.e., Vout_buffer min to Vout_buffer max) may beequal to, or substantially equal to, the input range of the buffer 120.

In various embodiments, the ADC 125 operates with a minimum referencevoltage Vrefl and a maximum reference voltage Vrefh. The maximum inputpixel voltage Vmax_pix may be higher than the maximum reference voltageVrefh, but the maximum output of the level-shifter 115 Vmax_level isless than or equal to the maximum reference voltage Vrefh. Additionally,the minimum input pixel voltage Vmin_pix and minimum level-shifter 115output voltage Vout_level_min may be greater than or equal to theminimum reference voltage Vrefl.

Referring now to FIGS. 4A-B, in operation, the voltage behavior of thereadout circuit 100 may be illustrated with transfer curves. Asillustrated in FIG. 4A, the transfer curve describes the voltage levelof the input signal as a function of time versus the output voltage ofthe buffer 120 as if the level-shifting circuit 115 was absent. Thegraph illustrates that the input voltage increases as a function of timeand the output voltage of the buffer also increases as a function oftime, wherein the voltage values have a first voltage range R1 from aminimum voltage Vmin to a maximum Vmax.

FIG. 4B illustrates a transfer curve describing the voltage level of theinput signal as function of time versus the output voltage thelevel-shifting circuit 115. The graph illustrates that the input voltageincreases as a function of time. Once the input voltage reaches apredetermined threshold voltage Vth, the level-shifter shifts thevoltage to a lower voltage, wherein the voltage values have a secondvoltage range R2. The second voltage range R2 may be defined as voltagevalues between the minimum voltage Vmin and an intermediate voltageVint, wherein the intermediate voltage Vint is less than the maximumvoltage Vmax. Subsequent signals with voltages above the thresholdvoltage Vth are also shifted to the second voltage range R2.

Referring now to FIGS. 5 and 7, in various embodiments, the readoutcircuit 100 comprises the storage device 105, the comparator 110, thebuffer 120, and the level-shifting circuit 115.

In various embodiments the readout circuit 100 may further comprisefirst and second switching devices S1, S2 to selectively couple variouscomponents of the readout circuit 100.

In various embodiments, the storage device 105 may comprise a top plateand a bottom plate to store electrical charge, for example a capacitor.The top plate of the storage device 105 is selectively coupled to theinput signal Vin_pix via the first switching device S1. The bottom plateof the storage device 105 is coupled to the level-shifting circuit 115.

In various embodiments, the level-shifting circuit 115 comprises thirdand fourth switching devices S3, S4. The third switching device S3 maybe selectively coupled to a first reference voltage Vref, while thefourth switching device S4 may be selectively coupled to a secondreference voltage Vref2. In the current embodiment, the third and fourthswitching devices S3, S4 may receive the comparator output signalVout_comp.

In various embodiments, the comparator 110 may be equipped with a latch505 for switching between a hold state and a comparing state. The latch505 may be responsive to a latch signal LATCH received from the controlunit. When the latch 505 is enabled (i.e., the latch signal LATCH is ahigh value), the comparator 110 is in the comparing state.Alternatively, when the latch 505 is disabled (i.e., the latch signalLATCH is a low value), the comparator 110 is in the holding state. Ingeneral, when the latch 505 is enabled, the comparator output Vout_compis continuously updated by the sign of the net differential inputsignal. Conversely, when the latch 505 is disabled, the comparatoroutput Vout_comp goes to either a logic “1” or a logic “0” depending onthe sign of the differential input signal at the instant of thetransition.

In various embodiments, the comparator 110 may transmit an output signalVout_comp to the level-shifting circuit 115.

Referring to FIG. 5, in one embodiment, the first switching device S1selectively couples the input signal Vin_pix to the storage device 105,and the second switching device S2 selectively couples the comparator110 and storage device 105 to the buffer 120.

Referring to FIG. 7, in an alternative embodiment, the first switchingdevice S1 selectively couples the input signal Vin_pix to the storagedevice 105, and the second switching device S2 selectively couples thestorage device 105 to the comparator 110 and to the buffer 120.

Referring to FIGS. 5 and 6, in one embodiment, during a first phaseφ1(1), the first and third switching devices S1, S3 receive a high value(the switches are “closed”). In the present embodiment, the input signalVin_pix is coupled to the top plate of the storage device 105, via thefirst switching device S1, while the bottom plate is coupled to thefirst reference voltage Vref via the third switching device S3. Thesecond and fourth switching devices S2, S4 receive a low value (theswitches are “open”) during the first phase φ1(1). In variousembodiments, the first switching device S1 may receive a control signalfrom the control unit for coupling the input signal Vin_pix to thestorage device 105. The voltage of the input signal Vin_pix is thensampled for a predetermined period of time.

During a second phase φ2(1), the first and third switching devices S1,S3 receive a low voltage value (“open”) and the latch 505 is enabled.The comparator 110 compares the voltage across the storage device 105 tothe threshold voltage Vth. If the voltage across the storage device 105is greater than the threshold voltage Vth, the comparator 110 transmitsthe digital value “1” (as illustrated with reference to the second phaseφ2(1) of a first sample 801) to the level-shifting circuit 115. If thedigital value “1” is received by the level-shifting circuit 115, thefourth switching device S4 receives a high value (“closed”), such thatthe bottom plate of the storage device 105 couples to the secondreference voltage Vref2, while the third switching device S3 maintains alow value (“open”). Coupling the bottom plate of the storage device 105to the second reference voltage Vref2 subtracts an offset value from thevoltage across the storage device 105. After the latch is disabled, thesecond switching device S2 receives a high value (“closed”) to couplethe storage device 105 to the buffer 120.

Alternatively, if the voltage across the storage device 105 is less thanthe threshold voltage Vth, the comparator 110 transmits the digitalvalue “0” (as illustrated with reference to a second phase φ2(2) of asecond sample 802) to the level-shifting circuit 115. If the digitalvalue “0” is received by the level-shifting circuit 115, then the fourthswitching device S4 remains at a low value (stays “open”), while thethird switching device S3 receives a high value (“closed”). The voltageacross the storage device 105 is input to the buffer 120 where thevoltage value is temporarily stored before it is transmitted to the ADC125.

In various embodiments, a first timing delay t_(delay1) may be presentbetween the rising edge of the latch signal LATCH and the comparatoroutput signal Vout_comp. Additionally, a second timing delay t_(delay2)may be present between the rising edge of the comparator output signalVout_comp and the rising edge of the switching device signals S3, S4.

In an alternative embodiment and referring to FIGS. 7 and 8, during afirst phase φ1(1), the first and third switching devices S1, S3 receivea high value (the switches are “closed”). In the present embodiment, theinput signal Vin_pix is coupled to the top plate of the storage device105, via the first switching device S1, while the bottom plate iscoupled to the first reference voltage Vref via the third switchingdevice S3. The second and fourth switching devices S2, S4 receive a lowvalue (the switches are “open”) during the first phase φ1(1). In variousembodiments, the first switching device S1 may receive a control signalfrom the control unit for coupling the input signal Vin_pix to thestorage device 105. The voltage of the input signal Vin_pix is thensampled for a predetermined period of time.

During a second phase φ2(1), the first and third switching devices S1,S3 receive a low voltage value (“open”), the latch 505 is enabled, andthe second switching device S2 receives a high value (“closed”) tocouple the storage device 105 to the comparator 110. The comparator 110compares the voltage across the storage device 105 to the thresholdvoltage Vth. If the voltage across the storage device 105 is greaterthan the threshold voltage Vth, the comparator 110 transmits the digitalvalue “1” (as illustrated with reference to a first sample 801) to thelevel-shifting circuit 115. If the digital value “1” is received, thefourth switching device S4 receives a high value (“closed”), such thatthe bottom plate of the storage device 105 couples to the secondreference voltage Vref2, while the third switching device S3 maintains alow value (“open”). Coupling the bottom plate of the storage device 105to the second reference voltage Vref2 subtracts an offset from thevoltage across the storage device 105.

Alternatively, if the voltage across the storage device 105 is less thanthe threshold voltage Vth, the comparator 110 transmits the digitalvalue “0” (as illustrated with reference to a second sample 802) to thelevel-shifting circuit 115. If the digital value “0” is received, thenthe fourth switching device S4 remains at low value (stays “open”),while the third switching device S3 receives a high value (“closed”).The voltage across the storage device 105 is input to the buffer 120where the voltage value is temporarily stored before it is transmittedto the ADC 125.

In various embodiments, a first timing delay t_(delay1) may be presentbetween the rising edge of the latch signal LATCH and the comparatoroutput signal Vout_comp. Additionally, a second timing delay t_(delay2)may be present between the rising edge of the comparator output signalVout_comp and the rising edge of the switching device signals S3, S4.

In the foregoing description, the technology has been described withreference to specific exemplary embodiments. The particularimplementations shown and described are illustrative of the technologyand its best mode and are not intended to otherwise limit the scope ofthe present technology in any way. Indeed, for the sake of brevity,conventional manufacturing, connection, preparation, and otherfunctional aspects of the method and system may not be described indetail. Furthermore, the connecting lines shown in the various figuresare intended to represent exemplary functional relationships and/orsteps between the various elements. Many alternative or additionalfunctional relationships or physical connections may be present in apractical system.

While the technology has been described with reference to specificexemplary embodiments, various modifications and changes may be madewithout departing from the scope of the present technology. Thedescription and figures are to be regarded in an illustrative manner,rather than a restrictive one and all such modifications are intended tobe included within the scope of the present technology. Accordingly, thescope of the technology should be determined by the generic embodimentsdescribed and their legal equivalents rather than by merely the specificexamples described above. For example, the steps recited in any methodor process embodiment may be executed in any order, unless otherwiseexpressly specified, and are not limited to the explicit order presentedin the specific examples. Additionally, the components and/or elementsrecited in any apparatus embodiment may be assembled or otherwiseoperationally configured in a variety of permutations to producesubstantially the same result as the present technology and areaccordingly not limited to the specific configuration recited in thespecific examples.

Benefits, other advantages and solutions to problems have been describedabove with regard to particular embodiments. Any benefit, advantage,solution to problems or any element that may cause any particularbenefit, advantage or solution to occur or to become more pronounced,however, is not to be construed as a critical, required or essentialfeature or component.

The terms “comprises”, “comprising”, or any variation thereof, areintended to reference a non-exclusive inclusion, such that a process,method, article, composition or apparatus that comprises a list ofelements does not include only those elements recited, but may alsoinclude other elements not expressly listed or inherent to such process,method, article, composition or apparatus. Other combinations and/ormodifications of the above-described structures, arrangements,applications, proportions, elements, materials or components used in thepractice of the present technology, in addition to those notspecifically recited, may be varied or otherwise particularly adapted tospecific environments, manufacturing specifications, design parametersor other operating requirements without departing from the generalprinciples of the same.

The present technology has been described above with reference to anexemplary embodiment. However, changes and modifications may be made tothe exemplary embodiment without departing from the scope of the presenttechnology. These and other changes or modifications are intended to beincluded within the scope of the present technology, as expressed in thefollowing claims.

The invention claimed is:
 1. A readout circuit, comprising: a storagedevice selectively coupled to an input signal and configured to samplethe input signal, wherein the input signal has a first voltage valuewithin a first voltage range; a comparator coupled to the input signal,wherein the comparator compares the first voltage value of the inputsignal to a predetermined threshold voltage; and a level-shiftingcircuit coupled to the storage device and responsive to the comparator,wherein the level-shifting circuit shifts the first voltage value of theinput signal to a second voltage value within a second voltage range ifthe first voltage value of the input signal is greater than thepredetermined threshold voltage.
 2. The readout circuit of claim 1,wherein the storage device comprises a top and bottom plate.
 3. Thereadout circuit of claim 2, wherein the level-shifting circuit comprisesa first switch to selectively couple the bottom plate of the storagedevice to a first reference voltage.
 4. The readout circuit of claim 3,wherein the level-shifting circuit comprises a second switch toselectivity couple the bottom plate of the storage device to a secondreference voltage to subtract an offset value from the input signal. 5.The readout circuit of claim 4, wherein the comparator transmits acontrol signal to the level-shifting circuit to couple the second switchto the second reference voltage.
 6. The readout circuit of claim 1,further comprising a buffer coupled to the storage device.
 7. Thereadout circuit of claim 1, further comprising a delta sigma modulatorcoupled to the level-shifting circuit and operative at the secondvoltage range, wherein the modulator converts the input signal into adigital signal.
 8. The readout circuit of claim 1, wherein thelevel-shifting circuit subtracts an offset value from the sampled inputsignal if the input signal is greater than the predetermined thresholdvoltage.
 9. The readout circuit of claim 1, wherein the comparatorprovides a most significant bit signal to a data formatting unitaccording to the input signal.
 10. A imaging system, comprising: a pixelarray comprising pixels arranged in rows and columns; a storage devicecomprising a top plate and a bottom plate, wherein the top plate iscoupled to the pixel array and samples the voltage of an input pixelsignal, and wherein the sampled input pixel signal has a first voltagerange; a comparator coupled to the storage device, wherein thecomparator compares the sampled input pixel signal to a predeterminedthreshold voltage; a level-shifting circuit coupled to the bottom plateof the storage device and responsive to the comparator, wherein thelevel-shifting circuit shifts the sampled input pixel signal to a secondvoltage range if the sampled input pixel signal exceeds thepredetermined threshold voltage; an analog-to-digital converter coupledto the level-shifting circuit and configured to convert the sampledinput pixel signal to a digital signal; and an output device coupled tothe analog-to-digital converter and configured to display an imageaccording to the digital signal.
 11. The imaging system of claim 10,wherein the level-shifting circuit comprises a first switch toselectively couple the bottom plate of the storage device to a firstreference voltage.
 12. The imaging system of claim 11, wherein thelevel-shifting circuit further comprises a second switch to selectivitycouple the bottom plate of the storage device to a second referencevoltage to subtract an offset value from the input signal.
 13. Theimaging system of claim 12, wherein the comparator provides a controlsignal to the second switch to couple the second switch to the secondreference voltage.
 14. The imaging system of claim 10, wherein, furthercomprising a buffer coupled between the storage device and theanalog-to-digital converter.
 15. The imaging system of claim 10, whereinthe comparator converts and transmits the most significant bit of theinput signal to a data-formatting unit.
 16. The imaging system of claim10, wherein analog-to-digital converter comprises a delta-sigmamodulator.